The invention relates to a laser Doppler anemometer with a wavelength-stabilized laser diode whose laser beam, after beam splitting, is concentrated in a certain area (measuring range) of a particle-containing flow of a fluid in such a way that an interference fringe pattern is produced in this area, and at least one photodetector for the detection of the two laser beams passing through and beyond the above-mentioned measuring range.
The invention starts from a laser Doppler anemometer as it is known from DE-OS No. 34 35 423. As compared with all other proven methods of laser Doppler anemometry, the present principle of measurement which remains to be described offers the advantage that the interaction between transmitter and receiver will be so optimized that at both the transmitting and receiving end the most inexpensive semiconductor components can be used even for extremely difficult measurement problems which have hitherto not been mastered. According to prior art, two partial beams of the same laser diode are superimposed in the area of a flowing fluid to be investigated in such a way that an interference fringe pattern is produced. If the wavelength of the laser diode is constant--which is to be ensured--, this interference fringe pattern will be stable and stationary.
The spacing of the intensity maxima of the interference fringe pattern is a function of the laser wavelength and the angle of intersection between the two laser beams in the measuring range.
A particle contained in a flowing fluid passing through this interference fringe pattern produces scattered light emissions with the intensity I.sub.scatter into the whole solid angle 4.pi. whose modulation frequency (Doppler frequency) is proportional to the particle velocity.
In the case of the conventional known LDA method, a receiving lens system focusses only a small part of the scattered light emitted into the whole solid angle 4.pi. on the photodetector which registers the Doppler frequency with more or less large signal-to-noise ratios depending on the amount of the scattered light power. When the Doppler frequency is determined by this known conventional LDA method, difficulties are particularly encountered when no sufficiently high scattered light intensity is registered, as the signal-to-noise ratios then become very small. This is particularly the case when the distance from the point of measurement is very great, as the apertures of the receiving lenses then confine the solid angle within which the scattered light is received, and when the particle sizes are of the order of micrometers or less, as these emit for physical reasons only a small amount of scattered light.
The idea underlying the new LDA method of measurement (4.pi. LDA) which is the object of the invention is to utilize for the receiving signals the whole scattered light power I.sub.scatter emitted into the solid angle 4.pi.. Accordingly, considerably better signal-to-noise ratios can be achieved than by the conventional LDA method, as only a fraction of the overall scattered light intensity contributes to the signal-to-noise ratio.
For the measurement of the flow velocity of a fluid also under the above-mentioned conditions (e.g. great distance from the point of measurement, smallest particles), with a simultaneous drastic cost reduction and miniaturization of an LDA measuring apparatus, the invention provides the determination of the velocity information directly from the differential intensity I between the laser beam intensity I.sub.o and the scattered light intensity I.sub.scatter emitted into the whole solid angle 4.pi., the relationship I=I.sub.o -I.sub.scatter being valid.